1. Field of the Invention
This invention relates to an information signal reproducing apparatus, and more particularly, to an apparatus for reproducing an information signal recorded in helical tracks on a tape-shaped record bearing medium.
2. Description of the Prior Art
The reproducing apparatus of the above-stated type is conceivable in varied kinds, including a magnetic recording and/or reproducing apparatus, an optical reproducing apparatus, etc. This specification, however, describes, by way of example, a video tape recorder (hereinafter referred to as a VTR) arranged to reproduce a video signal from a magnetic tape which has a television signal recorded thereon.
During recent years, high density recording has become possible permitting the development and the manufacture of a VTR capable of recording and reproducing over a long period of time with the pitch of helical recording tracks narrowed thereby allowing a magnetic tape to travel at a lowered speed.
Generally, each of these VTR's is arranged to permit switch-over between different operation time modes for recording and reproduction including, for example, a standard time mode, a long time mode, etc., so that the VTR can be operated also according to a conventional format. In reproducing from a magnetic tape, a signal recorded by the VTR which can be switched over between the different time modes, the magnetic tape must be played back in the same time mode that had been employed for recording. If not, the tracing locus of a reproducing head deviates from the center locus of the recording track formed on the tape resulting in a disturbed reproduced image. However, it would be very troublesome for the operator to manually switch over the reproducing time mode according to the reproduced image to avoid having such a disturbed image. The conventional solution of this problem has been as follow: A control signal (hereinafter referred to as a CTL signal) is recorded along the edge part of the magnetic tape at intervals of one frame during a recording operation. Then, during a reproducing operation, frequency generator detects the number of revolutions of a capstan motor which is proportional to the travelling distance of the magnetic tape, and produces an output (hereinafter referred to as an FG signal); any difference between the reproducing time mode and the recording time mode is detected by counting how many pulses of the FG signal are produced during one interval of the CTL signal, the reproducing time mode being thus automatically changed, accordingly. The details of an example of the conventional arrangement are as described below with reference to FIG. 1 of the accompanying drawings:
FIG. 1 shows the method for counting the pulses of the FG signal which represents the number of revolutions of a capstan motor in relation to the CTL signal. The illustration includes a magnetic tape 1 which is arranged to travel in the direction of an arrow by a capstan 2 and a pinch roller 3. A rotary magnetic head (not shown), which is mounted on a rotating drum as is well known, reproduces from the magnetic tape 1 a video signal which is recorded in helical tracks on the magnetic tape 1 in the longitudinal direction thereof. The capstan 2 is rotated at a predetermined speed by a capstan motor 4 through a belt 5. A frequency generator 6 detects the number of revolutions of the capstan motor 4. The number of pulses produced from this frequency generator 6 is in proportion to the travelling distance of the magnetic tape 1. The pulse signal of the generator 6 is supplied to a clock input terminal CK of a counter 8 via an amplifier 7. A control head 9 is arranged to reproduce the CTL signal which is recorded for every frame on the tape 1. The reproduced CTL signal is supplied, via an amplifier 10 and a waveform shaper 11, to the reset terminal R of the above-stated counter 8. Upon receipt of these inputs, the counter 8 counts the number of revolutions of the capstan motor 4 made during a period between one occurrence of reproduction of the reproduced CTL signal and another. The outputs Q1-Q4 of the counter 8 are supplied to a magnitude comparator 13. The comparator 13 is arranged to compare the output of the counter 13 with a reference value set and produced from a reference setter 12. The output Q5 of the comparator 13 is produced at a high level when the output of the counter 8 is either higher or lower than the reference value. Assuming that the number of pulses received by the counter 8 during one interval period of the CTL signal is 44 in case that recording has been performed in the standard time mode, the tape feeding speed is reduced to half in the event of recording in a long time mode which is twice as long as the standard time mode and the CTL signal interval period becomes 1/2 of the interval period obtained for recording in the standard time mode. Accordingly, when the magnetic tape which has been recorded in the long time mode is reproduced in the standard time mode, the counter receives during the CTL signal interval period only 22 pulses which are one half of 44. Therefore, the reference value of the reference setter 12 is preset at a value between 22 and 44. Then, the signal to be reproduced can be considered to have been recorded in the long time mode when the number of pulses counted by the counter 8 is less than the set reference value with reproduction made in the standard time mode; and to have been recorded in the standard time mode when the counted number of pulses is more than the set reference value.
However, there are some instances where no CTL signal is provided for indicating the pitch of the tracks on the tape-shaped record bearing medium, or where the CTL signal has dropped out of the medium. In that event, the track pitch cannot be found by the above-stated arrangement. The automatic mode switching operation described in the foregoing has been impossible, for example, for an apparatus of the kind arranged to perform tracking in accordance with a 4-f pilot method which has recently been proposed as will be described later herein.